Electrochemical devices for the detection of gases in fluid medium are important tools for the monitoring and control of various processes. For example, electrochemical sensors are utilized for air quality monitoring as well as for monitoring automobile exhaust and for controlling various industrial processes. The sensors generally operate to produce a signal, such as a current flow between electrodes, which is directly proportional to the concentration or the partial pressure of the gas being tested in the sample.
In the operation of electrochemical sensors, it is customary to periodically calibrate the zero and span performance of the sensor in order to re-confirm or re-establish the measurement accuracy. Calibration of the sensor elements is important in achieving accurate measurement with the sensor because sensors tend to have some zero drift over time in either a positive or negative direction. In accordance with conventional practice, the zero drift of the sensor element over time is compensated for or adjusted by periodic zero calibrations utilizing a known zero calibration gas composition which contains none of the gas being tested for. A similar problem exists with respect to the sensor span signal which gradually will change over a peiod of time due to aging of the sensor elements. This may similarly be corrected for or adjusted by periodic span calibrations of the sensor circuitry utilizing a calibration gas which contains a known quantity of test gas.
Calibration of the instrument can be a substantial operational problem since the instrument is normally taken off line and often removed to a suitable facility, such as a laboratory, for calibration where the calibration gas is introduced to the sensor elements. In many situations, such as in the case of industrial controls, the removal of the sensor from the control process may result in the loss of production time and considerable expense due to the process being normally shut down in the absence of control instrumentation. In the alternative, to avoid shut down of the process being monitored substitute instruments are used to replace the instruments being calibrated thus requiring a larger inventory of replacement instruments than required if the need to substitute instruments during the calibration process can be eliminated.
Methods and apparatus to automate the calibration process on line thereby eliminating the need for any substantial process shut down or equipment substitutions have been utilized, such as for example in U.S. Pat. No. 4,322,964 which provides a fluidic gate for the introduction of the calibration gas to an analyzer, or U.S. Pat. No. 4,489,590 which provides a method for automatically calibrating gas detectors which are monitoring combustible gases. However, such devices substantially add to the cost of the instrument and may not be suitable for many applications, such as for example, the monitoring of industrial atmospheres for toxic gases and the like.
Accordingly it would be desirable to provide low cost apparatus for easily and conveniently introducing calibration gases to sensors at their location without the necessity of moving the sensor or taking it out of service for any substantial period of time.